2D-to-3D localization refers to methods that recover the camera pose of a 2D image with respect to a 3D scene model. This 3D map is pre-built before performing global localization, via approaches such as structure from motion (SfM)[1]}. As shown in Figure REF , 2D-to-3D approaches establish 2D-3D correspondences between the 2D pixels of query image and the 3D points of scene model through local descriptor matching [2]}, [3]}, [4]} or by regressing 3D coordinates from pixel patches [5]}, [6]}, [7]}, [8]}. Such 2D-3D matches are then used to calculate camera pose by applying a Perspective-n-Point (PnP) solver [9]}, [10]} inside a RANSAC loop [11]}.
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